Electrodeposition of bright silver



United States Patent 3,446,716 ELECTRODEPOSITION OF BRIGHT SILVER Joachim Korpiun and Hans Joachim Steeg, Geislingen,

Steige, Germany, assignors to Dr. Ing. Max Schlotter, Geislingen, Steige, Germany N0 Drawing. Filed Sept. 27, 1966, Ser. No. 582,216 Claims priority, application Germany, Jan. 5, 1966, Sch 38,287 Int. Cl. C23b 5/26; C07c 153/05 US. Cl. 204-46 9 Claims This invention relates to the electrodeposition of bright silver coatings on conductive substrates, and particularly to novel brigtening agents and silver plating solutions containing the same.

We have found that silver is deposited cathodically from a cyanide bath in a. bright layer when the bath contains a sufiicient amount of a compound of the formula wherein Me is a metal selected to make the compound soluble in the plating bath.

The cyanide silver baths of the invention have excellent covering power so that bright, adherent deposits are readily formed over soft solder repairs in old silver plated pieces and other metallic substrates notoriously difficult to plate. Fully bright deposits are formed over a'wide range of current densities and the throwing power of the baths is high so that objects of very irregular shape can be provided with silver coatings of uniformly bright appearance. The silver deposits produced according to the invention retain their full brightness when covered with transparent lacquers, and do not require bufl'ing before lacquering.

The brighteners of the invention are effective in all known and commercially employed cyanide silver plating solutions, and variations in the concentration of the conventional ingredients within the usual limits recognized in this art have little, if any effect on the desirable properties of the solution mentioned above.

The nature of the cathodic component Me in the brightening agents of the invention is irrelevant if the metal employed does not unfavorably affect the silver deposit, the metals that may safely be added to silver plating solutions in the amounts inherent in this invention being well known.

The addition of 0.005 g. of a compound of the invention to one liter of a conventional silver plating bath based on potassium or sodium cyanide causes a readily visible brightening of the silver deposited from the bath. Fully bright deposits are formed over a current density range of 1 to 2 amps per square decimeter when the brightening agent is present in a concentration of at least 0.05 gram per liter, and the bright current densityrange can be widened by increasing the concentration to about 0.5 g. per liter. A nominal concentration of not more than one gram per liter is preferably restored from time to time by replenishment to maintain uniformity of the product. No measurable improvements can be obtained by increasing the concentration of the brightening agent beyond 5 grams per liter, but no unfavorable effects have been observed in cyanide silver plating baths containing more than 5 grams of the brightener per liter.

The current density range over which mirror bright deposits are produced in the silver plating baths of the 'invention may be widened materially by the use of auxiliary agents which do not in themselves produce bright deposits, but tend to suppress a bluish haze at current densities higher or lower than those at which the primary brightening agents of the invention are sulficient by themselves to provide full brightness.

Suitable auxiliary agents include the condensation products obtained from formaldehyde and naphthalenesulfonic acids by the method described in the German Patent 292,- 531 and in Houben-Weyls Handbook of Methods of Organic Chemistry (vol. XIV/2, pages 316, etc., of the current German edition). These condensation products are staple articles of commerce (Tamol NNO) and are believed to be mixtures of compounds of the formula HSO3 C1 H -(CH C H6SO3H) (see, for ex" ample, CA. 57, 8512f).

Other sulfated organic acids and esters may be employed for the same purpose, such as saponified Turkeyred oil and sulfated esters of oleic acid, but none has been found to be equally effective at the same concentration as the formaldehydenaphthalenesulfonic acid condensation products mentioned above.

The bright plating range of a cyanide silver bath containing the brighterners of the invention can be extended to higher current density values by relative movement of the object to be plated and of the plating solution, as by cathode rod agitation or in any other conventional manner.

The following examples further illustrate the invention, but it will be understood that the invention is not limited thereto.

EXAMPLE 1 620 g. potassium hydroxide was dissolved in 4 liters methanol, and the solution was stirred at room temperature while 1000 g. thiosemicarbazide and thereafter 900 g. carbon disulfide were gradually added. The mixture was refluxed for six hours and was then left to stand for 24 hours at ambient temperature. The slurry formed was filtered with suction, and the crystalline precipitate was washed repeatedly with acetone and ultimately dried.

It was identified as the compound A silver plating solution of the following composition was prepared:

G./l. Silver, as silver cyanide 30 Free potassium cyanide 125 Compound A 0.1

Mirror bright silver deposits were obtained on the cathode at 25 C. over a current density range from 0.8 to 1.6 amps per square decimenter, Deposits produced at somewhat higher cathode current densities showed a slight bluish haze. Brightness decreased visibly at lower current densities.

' EXAMPLE 3 -An electrolyte was made up to the following composition:

G./l. Silver, as silver cyanide 25 Free sodium cyanide Compound A 0.06

EXAMPLE 4 A silver plating solution was prepared from the following compounds:

G./l. Silver, as silver cyanide 30 Free potassium cyanide 140 Compound A 0.15

Condensation product of naphthalenesulfonic acids and formaldehyde When operated at temperatures between 22 and 25 C., the solution yielded mirror bright silver deposits free from haze over a current density range of 0.1 to 4 amps per square decimeter. Cathode rod agitation or other provisions for relative movement of the solution and the objects to be plated were necessary at current densities above 1.5 amps per square decimeter for best results. Full coverage of the plated objects with a uniformly bright silver layer was achieved over the entire current density range within ten to twenty seconds. The mirror brightness of the silver deposits was not affected by lacquering with a transparent, heat-curing lacquer and by subsequent baking of the lacquer coating.

EXAMPLE 5 Results similar to those described in Example 4 were obtained with an electrolyte of the composition:

Silver, as silver cyanide 40 Free sodium cyanide 100 Compound A 0.075 Condensation product of naphthalenesulfonic acids and formaldehyde 3.0

Fully bright silver deposits were produced at 22 to 25 C. at a primary current density of 1.5 to 2.0 amps per sq. decimeter with agitation from a bath of the following composition:

G./l. Silver, as silver cyanide 30 Free potassium cyanide 160 Compound A 0.3 Saponified Turkey-red oil 2 The temperatures indicated in Examples 2 to 6 were determined by the equipment employed and by the prevailing ambient temperatures. They are not critical. Higher temperatures are generally undesirable in silver plating, as is well known, but do not significantly affect the performance of the brightening agents of the invention. When operating temperatures below 22 C. are maintained, the entire bright plating range is shifted to lower current densities. The width of the range remains virtually unchanged.

The salts of the alkaline earth metals analogous to Compound A can be prepared in a manner obvious from Example 1, and are equally effective. They are converted to the sodium or potassium salts in the plating solution, and the corresponding alkaline earth metal carbonates are precipitated by the alkali metal carbonates unavoidably present in alkaline solutions open to the atmosphere.

Such precipitates are generally undesirable, and may have to be removed by filtration before deposits of highest brightness can be produced. Similar considerations militate against the use of other soluble salts in which Me is a metal other than an alkali metal.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not limited thereto, but is to be construed broadly and restricted solely by the scope of the appended claims.

What is claimed is:

1. A compound of the formula wherein Me is a metal, the compound being soluble in an aqueous solution of silver cyanide and free alkali metal cyanide to a concentration of about 5 grams per liter.

2. A compound as set forth in claim 1, wherein Me is alkali metal.

3. A compound as set forth in claim 2, wherein Me is potassium.

4. A silver plating electrolyte essentially consisting of an aqueous solution of silver cyanide and of free alkali metal cyanide, and containing as a brightening agent an effective amount of a compound of the formula wherein Me is a metal, said compound being dissolved in said aqueous solution.

5. An electrolyte as set forth in claim 4, wherein the concentration of said brightening agent is between 0.005 g/l. and 5 g./l.

6. An electrolyte as set forth in claim 5, wherein Me is an alkali metal.

7. An electrolyte as set forth in claim 6, wherein Me is potassium.

8. An electrolyte as set forth in claim 6, further containing an effective amount of an auxiliary agent selected from the group consisting of condensation products of naphthalenesulfonic acids with formaldehye and saponified Turkey-red oil.

9. A method of coating a conductive object with a bright deposit of silver which comprises making said object the cathode in an electrolyte essentially consisting of an aqueous solution of silver cyanide and of free alkali metal cyanide, and containing as a brightening agent an effective amount of a compound of the formula HzN-fi-NH-NH-fi-S-Me wherein Me is a metal, said compound being dissolved in said aqueous solution.

References Cited UNITED STATES PATENTS 2,730,492 1/ 1956 Chester 204-44 2,800,439 7/1957 Fischer et al. 204-46 XR 3,198,698 8/1965 Reuter et a1. 260-5135 XR 3,219,558 11/1965 Foulke 204-46 PATRICK P. GARVIN, Primary Examiner.

US. Cl. X.R. 

4. A SILVER PLATING ELECTROLYTE ESSENTIALLY CONSISTING OF AN AQUEOUS SOLUTION OF SILVER CYANIDE AND OF FREE ALKALI METAL CYANIDE, AND CONTAINING AS A BRIGHTENING AGENT AN EFFECTIVE AMOUNT OF A COMPOUND OF THE FORMULA 